Improve Chapter 01 specialization lesson

[jules-tth]

Summary

• Replace the placeholder specialization sublesson with an operator-focused map of Linux-adjacent disciplines.
• Cover core operations, networking, security, cloud infrastructure, containers, automation, databases, SRE, and developer tooling.
• Add escalation boundaries, a handoff-note template, a practical incident-mapping lab, and review questions.

Verification

• python3 -m mkdocs build --strict
• git diff --check
• find docs -type f -name "*.md" -size 0 -print
• make build

Duplicate check

• PR Improve Chapter 01 hosting options lesson #310 already covers the prior planned 1.11 hosting-options lesson, so this slice skipped to 1.12.
• Final open-PR file-overlap check for docs/lessons/01_where_to_start/1.12_specialization_and_adjacent_disciplines.md returned no matches immediately before PR creation.